1. Field of the Invention
The present invention relates to a method for controlling delivery of fluid from a piston pump which is provided with automatic intake and discharge valves and to a device which can successfully accomplish the method.
2. Description of the Prior Art
Piston pumps, or pumps with positive displacement as they are also called, are employed in several fields in industry and technology where high delivery pressure is required. Such pumps are usually provided with automatic valves wherein the intake and discharge valves are opened by the diminished pressure and increased pressure respectively formed in the pump cylinder during reciprocal movement of the piston therein.
Since the quantity of liquid displaced by the piston of a piston pump during its working stroke is only to a small extent dependent upon the working conditions of the pump, the working pace of the pump, e.g. its shaft speed, must usually be adjustable if it is desired to vary the quantity of liquid delivered from a given pump. In the case of small pumps, especially for the injection of fuel into diesel engines, a known method of regulating the delivery of the pump is to keep the intake valve open during part of the working stroke of the piston so that some of the liquid drawn into the pump cylinder is returned to the intake duct during the working stroke. This can be achieved, for example, by allowing the intake valve to consist of an aperture in the pump cylinder wall which is closed by the pump piston at a given point of time during the working stroke. The closing time can be varied by turning the piston, a cavity in its side wall being arranged so that at the rear it is limited by a surface at an angle to the longitudinal axis of the piston. During the forward movement of the piston, liquid will thus flow out through the inlet aperture via the cavity until its sloping surface has passed the inlet aperture, so that a corresponding proportion of the piston's pumping movement is ineffective. In this system the piston must be rotatively arranged in the cylinder, and rotation of the piston should preferably be controllable from the outside while the pump is running. Moreover, close tolerances are required between piston and cylinder wall in order to achieve sufficient sealing of the intake aperture, and the cavity in the piston makes it impossible to employ sealing piston rings. This system is therefore unsuitable for large piston pumps and for the pumping of liquids with poor lubricating properties.
Another quantity regulation system is known in small pumps, in which the inlet aperture is closed by a moveable valve body. In these pumps a valve body is caused to open after the pump piston has completed part of its forward movement so that contents of the pump cylinder are returned to the inlet duct during the remaining part of the piston stroke. The valve body is moved by a mechanical lifting device which is operated by a variable cam mechanism of relatively complex design. In view of the fact that the valve body has to be moved against the full pressure differential of the pump, the lifting mechanism must be capable of exerting considerable force, which implies costly production and considerable exposure to wear. In addition, the system requires a relatively large amount of energy.
In drilling for natural resources such as, e.g. oil and gas, drilling mud is pumped down through the hollow drill pipe for the purpose of cooling and cleaning the crown and to carry drilling spoil out of the borehole. The drilling mud must often be supplied at very high pressures, and the flow required varies during the drilling operation. Large piston pumps are used for this drilling mud injection, their delivery being regulated by varying the pace of the pump, i.e. its shaft speed.
Where drilling is carried out in a marine environment, e.g. from a floating platform installation, it is usual to use direct current motors to drive the drilling mud pumps in order to be able to vary the pump shaft speed. A direct current motor necessitates highly complicated and sensitive control equipment which requires a great deal of maintenance and a large stock of spares. Besides being costly, the direct current motor and its equipment require considerable space and it is of great weight, thus undesirably affecting the payload of the platform.
The purpose of the present invention is thus to provide a method for the regulation of delivery from a piston pump of the type mentioned by way of introduction, which permits the use of a lighter, less expensive and more rugged alternating current motor for the operation of the pump, while at the same time wholly or partly eliminating the disadvantages of known pumps. Furthermore, it is the purpose of the invention to provide a device to carry out the method.
For a better understanding of the invention it will now be described in detail below with reference to the embodiment shown in the accompanying drawings.